Current limiting battery feed arrangement

ABSTRACT

A current limiting circuit in a battery feed arrangement provides a variable DC resistance for limiting subscriber loop current to a threshold value. The current limiting circuit also presents a low AC impedance so that no imbalance is introduced between tip and ring leads of the subscriber loop. The current limiting component is a low ON resistance power MOSFET in the ring side of the battery feed arrangement. The current limiting circuit is self protecting in the event of lightning strikes and AC power signals either induced or short-circuited.

This application is a continuation-in-part of Application Ser. No.07/684,784, filed Apr. 15, 1991.

This invention relates to battery feed for a telephone subscriber loopinterface and is particularly concerned with limiting loop current.

BACKGROUND OF THE INVENTION

Most telephone sets draw DC current from the subscriber loop thatconnects them to the central office (CO). For satisfactory operation,the telephone set requires a current in the range of 18 mA to 50 mA. TheDC loop current is provided by the central office battery, whoseterminals are typically coupled to the subscriber loop via two batteryfeed resistors. The DC resistance measured between the TIP and RING ofthe subscriber loop (including the telephone set) typically is in therange of 100 to 1900 ohms and depends upon the length of the subscriberloop. To provide sufficient loop current for telephone operation withthe longest subscriber loop, the value of the battery feed resistors istypically limited to 200 ohm each. In this case, the loop current isI_(loop) =52V/(400+1900)=22.6 mA, which is above the minimum currentrequired for proper operation. In the case of a short subscriber loop,one having a resistance of 100 ohm, the loop current is I_(loop)=52V/(400+100)=104 mA, which exceeds the desired range of loop currentfor proper telephone set operation.

It is desirable to provide a battery feed arrangement capable oflimiting the loop current in the case of a short subscriber loop. Acircuit to limit the loop current would have to meet the followingrequirements: provide variable DC resistance to limit the loop current;provide low AC impedance to meet the voice frequency requirements of thesubscriber line interface; and withstand, without further protection, ACsignals appearing at the TIP and RING and resulting from lightningstrikes on the loop, AC power conductor induction, and AC powerconductor short-circuit.

There have been attempts to introduce current limit circuits with thebattery-feed. For example, John M. Hurt in U.S. Pat. No 4,560,834 issuedDec. 24, 1985, teaches an active current limiter in only the ring sideof a telephone circuit. While the arrangement taught by Hurtsuccessfully limits loop current to the desired range, the presence ofcertain components within the arrangement may prevent it from meetingall of the requirements listed above.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved currentlimiting battery feed arrangement.

In accordance with the present invention there is provided a batteryfeed arrangement for a telephone subscriber loop comprising: inputs forconnection to power supply terminals; outputs for connection to asubscriber loop; a first battery feed resistor connected between one oinput and one output; a second battery feed resistor, having a valueless than the first battery feed resistor, connected to the otheroutput; a power FET having a control terminal and a controllable DCresistive path series connected with the second battery feed resistorand coupled to the other input; a capacitor connected between thecontrol terminal and the resistive path of the power FET for a lowimpedance path for AC signals applied at the other output; and a controlcircuit connected to the control terminal of the power FET and having aloop current sensing subcircuit and a biasing subcircuit for providing acontrol voltage to the power FET in dependence upon the loop current;the control voltage having a first value for maintaining thecontrollable resistive path in a low resistance state when the loopcurrent is below a predetermined threshold and a second value forincreasing the controllable resistive path to a higher resistance stateto maintain the loop current at the predetermined threshold.

Advantages of the present invention are a variable DC resistance tolimit subscriber loop current and a low AC impedance to meet the voicefrequency requirements of the subscriber line interface and an abilityto withstand AC signals appearing at connections to the subscriber loopdue to lightning strikes, AC induction and AC power shortcircuits,without further protection.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further understood from the followingdescription with reference to the drawings in which:

FIG. 1 illustrates a known battery feed arrangement including a currentlimiting circuit;

FIG. 2 schematically illustrates a battery feed arrangement including acurrent limiting circuit in accordance with a first embodiment of thepresent invention;

FIG. 3 graphically illustrates the impedance of the current limitingcircuit of FIG. 2 as a function of complex frequency; and

FIG. 4 schematically illustrates a battery feed arrangement inaccordance with a second embodiment of the present invention.

Similar references are used in different figures to denote similarcomponents.

DETAILED DESCRIPTION

Referring to FIG. 1, there is illustrated a known battery feedarrangement having a current limiting circuit in the ring side. Thecircuit of FIG. 1 is taught by J. M. Hurt in U.S. Pat. No. 4,560,834,issued Dec. 24, 1985. As in typical battery feed arrangements, thecentral office includes a line interface circuit 1 for interconnectingthe line 2 with the subscriber loop 3. The line 2 terminates in awinding 4 of a transformer 5. Tip and Ring sides of loop 3 are connectedto opposite halves 6a and 6b of a split winding 6 of transformer 5. Thetwo winding halves 6a and 6b are joined together by a capacitor 7. Thejunction of capacitor 7 and winding half 6a is connected to groundthrough a 200 ohm resistor 8 and the junction of capacitor 7 and windinghalf 6b is typically connected to a -52 volt battery through a further200 ohm resistor 9. However, in FIG. 1 a current limiter 10 is placedbetween the -52 volt battery and the resistor 9. A comparator 12 hasinputs connected to both sides of the battery feed resistors 8 and 9 andan output connected to a control circuit 14. The control circuit 14 isconnected to the current limiter 10 and an auto balance circuit 16.

The current limiter 10 includes a bias circuit 18 having an inputconnected to the auto balance circuit 16 and two outputs. One output ofthe bias circuit 18 is connected to a transistor 20 via its emitter andits base is connected to the control circuit 14 via a lead 22. Thetransistor 20 is connected to the -52 volt battery via its collector, adiode 24, a resistor 26 and a zener diode 28. The zener diode 28 isconnected to the battery feed resistor 9 via a zener diode 30 and a lead32. The lead 32 is also connected to a diode 34 and a capacitor 36,parallel connected to ground, and to a transistor 38 via its emitter. Atransistor 40 has its base connected to the collector of transistor 20,its emitter connected to the collector of transistor 38 and itscollector connected to the base of transistor 38. The emitter oftransistor 40 and the collector of transistor 38 are connected via aresistor 42 to the zener diode 28. The other output of the bias circuit18 is connected via a lead 44 to a node of the lead 32 and the batteryfeed resistor 9. The control circuit 14 is connected to the auto balancecircuit 16 via a lead 46.

In operation, the current limiter 10 maintains the loop current withinthe desired range of 18 mA to 50 mA. However, several of the componentsused in the current limiter may cause problems in actual telephonesubscriber loop application. Firstly, when the subscriber loop is in the`ON-HOOK` state the diode 28 and the transistor 38 contribute to avoltage drop of 1.2 volts between the -52 volt battery input and the Rlead. This is not compatible with maintenance terminating units (MTU)which are widely used in telephone loops. The MTU requires a voltage of-42.75 volts on the R lead at a battery voltage of -42.75 volts. Thecircuit of FIG. 1 would provide a voltage of -42.75+1.2=-41.55 volts.Secondly, with the components used, is that an AC current induced to theR lead is rectified by the diode 28 and the transistor 38 to produce adischarge for the central office battery. Finally, the node between thelead 32 and the battery feed resistor 9 is not low impedance when thereis no current flowing, as in the `ON-HOOK` state. This may make thecircuit unsuitable for voice frequency (VF) transmission in the`ON-HOOK` state, which is a widely used signalling technique.

Further, the current limiter 10 of FIG. 1 is not self-protecting againstlightning strikes and AC surges, rather it uses protection in the formof zener diodes 28 and 30 and diode 34.

Thus the circuit of FIG. 1, while meeting the first criterion set forthhereinabove, may not meet the second two, namely, provide a low ACimpedance and withstand, without further protection, lightning strikesand AC surges.

Referring to FIG. 2, there is schematically illustrated a battery feedarrangement in accordance with a first embodiment of the presentinvention. The central office includes a line interface circuit 10,which in common with the prior art circuit of FIG. 1, includes atransformer 5 with winding 4 connected to the line 2, split windings 6aand 6b connected to the TIP and RING sides respectively of thesubscriber loop 3 and a capacitor 7 connected between windings 6a and6b.

As with the prior art arrangement, the junction of winding 6a andcapacitor 7 is connected to ground through a 200 ohm resistor 8.However, the 200 ohm resistor 9 and current limiter 10 of the priorarrangement are replaced with a current limit circuit in series with aloop feed resistor 50 having a resistance of 187.6 ohms.

The current limit circuit includes a low ON resistance power MOSFET 52and a control arrangement coupled thereto. The control arrangementincludes a 12.1 ohm current sensing resistor 54, a low pass filterhaving 150 kohm resistors 56 and 58 and a 0.1 μF capacitor 60, a bipolartransistor 62, and a voltage divider including resistors 64 and 66. Thedrain of the power MOSFET 52 is connected to the resistor 50 and thesource of the power MOSFET 52 is connected through the 12.1 ohm currentsensing resistor 54 to the -52 volt battery. The low pass filter,comprising 150 kohm resistors 56 and 58 and a 0.1 μF capacitor 60, isconnected between the source of FET 52 and the base of bipolartransistor (or small signal FET) 62 connected to control the gate of FET52. DC bias for the gate of FET 52 is provided by the 150 kohm resistor64 connected effectively between the gate and -52 volt battery and the410 kohm resistor 66 connected between the gate and the 200 ohm resistor8. A 0.1 μF capacitor 68 is connected between the drain and gate of FET52.

In operation, with no loop current flowing, i.e., subscriber's telephoneis `ON-HOOK` the power FET 52 is held ON, in a saturated mode, by a biasvoltage of approximately 10 volts across the resistor 64. The power FET52 has an ON resistance of approximately 0.3 ohm, hence the totalresistance on the ring side of the battery feed circuit, provided byresistors 50 and 54 and the power FET 52, is 200 ohm and is balancedwith respect to the tip side whose resistance of 200 ohm is provided bythe resistor 8. As the saturated power FET 52 appears as a smallresistance to both DC and AC signals, lightning strikes on the loop andAC power signals in the loop, either induced or due to short-circuits topower conductors, do not damage the power FET during the typically shorttime period required for subscriber loop overvoltage protection to takeeffect.

With a loop current flowing, i.e., subscriber's telephone is `OFF-HOOK`,a voltage drop develops across the resistor 54. If the loop currentremains within the normal range of 18 mA to approximately 50 mA, thevoltage drop across the resistor 54 remains below Vbe=0.6 V for thetransistor 62 and the power FET 52 remains in the saturated mode. If theloop current attempts to exceed a predetermined threshold value, chosento be 49.5 mA in the present embodiment, which is approximately theupper end of the normal loop current range, the voltage drop across theresistor 54 reaches V_(be) =0.6 volts for the transistor 62, whichbegins to be turned on. The turning on of transistor 62 reduces the gatevoltage of power FET 52 causing its DC resistance to increase tomaintain a loop current of 49.5 mA, in which case the battery feedcircuit operates in a current limiting mode. In case of a short cable(100 ohms) the loop current would be:

    I.sub.loop max=52V/(100+400)=104 mA The transistor 62 increases the source to drain resistance of FET 52 to approximately 550 ohms, thus limiting the loop current to

I_(loop) =0.6V/12.1=49.5 mA.

Capacitor 60 filters the AC components of the signal fed to the base oftransistor 62.

While the current limiting circuit controls the DC current by increasingthe DC resistance of the power FET 52, its AC impedance remains low asis explained hereinbelow in reference to FIG. 3.

Referring to FIG. 3, there is graphically illustrated the impedance ofthe ring side of the battery feed circuit in current limiting mode as afunction of complex frequency s. A curve 70 represents the drain tosource impedance Z_(ps) which for relatively low frequency signals (lessthan 4 kHz) is given by equation 1:

    Z.sub.DS =(1+1/(SR'.sub.64 C.sub.68))/g.sub.m (EQ. 1)

where R'64=R₆₄ and R₆₆ in parallel=109 kΩ, C₆₈ =0.1 μF and thetransconductance of FET 52 is g_(m) =3 mhos. The curve 70 representingthe impedance has two asymptotes 72 and 74. The asymptote 74 issubstantially horizontal and represents an impedance of about 0.33 ohm.The asymptotes 72 and 74 intersect at a frequency f₁ approximately equalto 15 Hz. Thus, in the current limiting mode the battery feed circuitpresents a maximum current limiting resistance of 550 ohm to the DC loopcurrent and while maintaining a low impedance of about 0.33 ohm to anyAC signal above the cutoff frequency f₁ of about 15 Hz. The capacitor 68and resistors 64 and 66 are selected to give the cutoff frequency ofapproximately 15 Hz. Thus, in the current limiting mode, the batteryfeed circuit is current limiting for DC, balanced for voice frequencyand AC power signals, and provides a low impedance to lightning and ACpower signals. An example of a suitable power FET is MTP10N15 byMotorola.

Referring to FIG. 4, there is schematically illustrated a battery feedarrangement in accordance with a second embodiment of the presentinvention. The central office includes a line interface circuit 10,which in common with the prior art circuit of FIG. 1 and the embodimentof FIG. 2, includes a transformer 5 with winding 4 connected to the line2, split windings 6a and 6b connected to the TIP and RING sidesrespectively of the subscriber loop 3 and a capacitor 7 connectedbetween windings 6a and 6b.

As with the prior art arrangement, the junction of winding 6a andcapacitor 7 is connected to ground through a 200 ohm resistor 8.However, the 200 ohm resistor 9 and current limiter 10 of the priorarrangement are replaced with a current limit circuit in series with aloop feed resistor 70 having a resistance of 199.7 ohms.

The current limit circuit includes a low ON resistance power MOS FET 52the drain of which is connected to resistor 70 and the source of whichis connected to the -52 volt battery. The 0.1 BF capacitor 68 isconnected between the drain and gate of FET 52 as in the embodiment ofFIG. 2.

The second embodiment uses a different control arrangement forcontrolling the power FET 52 from that of the first embodiment of FIG.2. Two voltage dividers including resistors 72 and 74 and resistors 76and 78, respectively, are connected between the line side of batteryfeed resistor 70 and the ground side of battery feed resistor 8 and thebattery side of battery feed resistor 70 and the line side of batteryfeed resistor 8, respectively. A first operational amplifier 80 has anoninverting input connected to the junction of resistors 72 and 74 andan inverting input connected to the junction of resistors 76 and 78. Thenoninverting input of operational amplifier 80 is connected to -52Vbattery via a resistor 82. The inverting input of operational amplifier80 is connected to its output via a resistor 84. A second operationalamplifier 86 has an inverting input connected to the output ofoperational amplifier 80, and a noninverting input connected to areference potential. The reference potential is provided at the junctionof a resistor 88 connected to ground and a zener diode 90 connected to-52V battery. The output of operational amplifier 86 is connected to thegate of power FET 52 via a resistor 92.

In operation, a voltage signal proportional to the loop current isgenerated by the operational amplifier 80 and voltage divider resistors72, 74, 76 and 78 at a node 94. In the embodiment of FIG. 4, the scalingfactor for operational amplifier 80 is K=0.1 V/mA of loop current. Thesignal output by operational amplifier 80 is compared to the referencevoltage by operational amplifier 86. The output of operational amplifier86 controls the gate to source voltage of power MOSFET 52 and as theoverall feedback is negative, the following equation is forced to betrue by the circuit: ##EQU1##

Thus, the loop current I_(loop) is limited to 40 mA. The current limitvalue is a design parameter that may be changed by changing eitherV_(ref) or K.

The cutoff frequency, f₁, in the second embodiment is determined by thetime constant for the capacitor 68 and the resistor 92. For C₆₈ =0.1 μFand R₉₂ =200 k the time constant provides a cutoff frequency of about 20Hz.

The remainder of the circuit features are as described with regard tothe embodiment of FIG. 2.

Numerous modifications, variations and adaptations may be made to theparticular embodiments of the invention described above withoutdeparting from the scope of the invention, which is defined in theclaims.

What is claimed is:
 1. A battery feed arrangement for a telephonesubscriber loop comprising:inputs for connection to power supplyterminals; outputs for connection to a subscriber loop; a first batteryfeed resistor connected between one input and one output; a secondbattery feed resistor, having a value less than the first battery feedresistor, connected to the other output; a power FET having a controlterminal and a controllable DC resistive path series connected with thesecond battery feed resistor and coupled to the other input; a capacitorconnected between the control terminal and the resistive path of thepower FET for a low impedance path for AC signals applied at the otheroutput; and a control circuit connected to the control terminal of thepower FET and having a loop current sensing subcircuit and a biasingsubcircuit for providing a control voltage to the power FET independence upon the loop current; the control voltage having a firstvalue for maintaining the controllable resistive path in a lowresistance state when the loop current is below a predeterminedthreshold and a second value for increasing the controllable resistivepath to a higher resistance state to maintain the loop current at thepredetermined threshold.
 2. A battery feed arrangement as claimed inclaim 1 wherein the loop current sensing subcircuit includes a currentsensing resistor in series with the power FET.
 3. A battery feedarrangement as claimed in claim 2 wherein the biasing circuit includes abipolar transistor controlling the gate of the power FET.
 4. A batteryfeed arrangement as claimed in claim 3 further comprising a low passfilter connected between the other input and the base of the bipolartransistor.
 5. A battery feed arrangement as claimed in claim 1 whereinthe predetermined threshold of the loop current is in the range 18-50mA.
 6. A battery feed arrangement as claimed in claim 1 wherein the loopcurrent sensing subcircuit includes first and second voltage dividersconnected diagonally across the first and second battery feed resistors.7. A battery feed arrangement as claimed in claim 6 wherein the loopcurrent sensing subcircuit includes an amplifier connected to the firstand second voltage dividers.
 8. A battery feed arrangement as claimed inclaim 7 wherein the biasing subcircuit includes an amplifier having aninverting input and a noninverting input, the inverting input connectedto the output of the loop current sensing amplifier and the noninvertinginput connected to a reference potential.